Controlled-source electromagnetic (“CSEM”) geophysical surveys use man-made sources to generate electromagnetic fields to excite the earth, and deploy receiver instruments on the earth's surface, on the seafloor, in the air, or inside boreholes to measure the resulting electric and magnetic fields, i.e., the earth's response to the source excitation. FIG. 1 illustrates the basic elements of an offshore CSEM survey. A vessel tows a submerged CSEM transmitter 11 over an area of subseafloor 13. The electric and magnetic fields measured by receivers 12 are then analyzed to determine the electrical resistivity of the earth structures beneath the surface or seafloor. This technology has been applied for onshore mineral exploration, oceanic tectonic studies, and offshore petroleum and mineral resource exploration. See A. D. Chave, S. Constable, and R. N. Edwards, in Electromagnetic Methods in Applied Geophysics (ed. M. N. Nambighian), Vol. 2, 931-966, Society of Exploration Geophysicists; L. MacGregor, M. Sinha, and S. Constable, Geophy. J Int. 146, 217-236 (2001); T. Eidesmo, S. Ellingsrud, L. M. MacGregor, S. Constable, M. C. Sinha, S. Johansen, F. N. Kong, and H. Westerdahl, First Break 20.3, 144-152 (2002).
The conventional method of displaying electromagnetic data for interpretation is to show the amplitude of the electric field versus offset (distance between transmitter and receiver). As FIG. 2 illustrates, the picture becomes very confusing if the data corresponding to all receivers of the same tow line are displayed together.
Another data display method consists in selecting a constant offset and displaying, in map view, the ratio or the difference between the observed amplitude at this offset and a reference amplitude (S. Ellingsrud, et al., The Leading Edge 21 972-982, (October, 2002)). The selection of one unique offset may not be optimal, and valuable information is lost if one cannot look at the whole-offset curve.
MacGregor and Sinha disclose presenting the results of a CSEM survey using normalized values wherein the observed field is divided by the field calculated for a reference model. Alternatively, they state that normalization could be based on the survey data itself, using data collected adjacent to the target. They suggest this technique for allowing clearer visualization of effects of buried structures on CSEM signal properties, effects that might otherwise be less clear because the signal amplitude in a typical survey is likely to vary by several orders of magnitude over the useful set of offsets. (PCT International Publication No. WO 03/048812, Jun. 12, 2003)